Bacterial nitroreductase for the reduction of CB1954 and analogues thereof to a cytotoxic form

ABSTRACT

A prodrug of the formula (I): ##STR1## where R 1  is a group such that the compound R 1  NH 2  represents actinomycin D, doxorubicin, mitomycin C, or a nitrogen mustard of the formula (IV): ##STR2## a prodrug of the formula (II): ##STR3## where R 2  is a group such that the compound R 2  OH is a phenolic nitrogen mustard; and processes whereby the prodrugs shown above are made in which a prodrug precursor compound is reacted with 4-nitrobenzyl chloroformate under anhydrous conditions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 08/504,047, filed Jun. 7, 1995, U.S. Pat. No. 5,780,585 which is a division of U.S. application Ser. No. 08/232,018, filed Jul. 27, 1994, U.S. Pat. No. 5,633,158 which is the U.S. National Stage of International Application No. PCT/GB92/01947 filed Oct. 23, 1992, the contents of which are incorporated herein by reference.

THIS INVENTION relates to the control of neoplastic tissue growth and is particularly concerned with the provision of new anti-tumour agents and with enzymes capable of converting prodrugs into anti-tumour agents.

The alkylating agent 5-(aziridin-1-yl)-2,4-dinitrobenzamide (hereinafter designated CB 1954) has been known, almost for 20 years, as an interesting experimental compound of unique selectivity. Although CB 1954 is structurally quite closely related to numerous other known alkylating agents which have a relatively broad range of activity, CB 1954 exhibits considerable activity against the Walker tumour cells in vivo or in vitro but was thought to be virtually inactive against other tumours.

It was recently discovered that the selectivity of CB 1954 arose from the fact that it was not an anti-tumour agent per se but was a prodrug for an anti-tumour agent generated from CB 1954 by a nitroreductase enzyme found in the Walker cell. This nitroreductase from the Walker cell was subsequently shown to be an enzyme known from other sources which was an NAD(P)H dehydrogenase (quinone) classified as EC.1.6.99.2, see Robertson et al, J. Biol. Chem. 261, 15794-15799 (1986).

In the course of the previous investigations with CB 1954, it was found that the Walker cell enzyme EC.1.6.99.2 had the ability to reduce the 4-nitro group of CB 1954 to the corresponding hydroxylamine and that it was the resulting 5-(aziridin-1-yl)-2-nitro-4-hydroxylamino-benzamide that was the active anti-tumour agent.

The use of prodrugs represents a clinically very valuable concept in cancer therapy since, particularly where the prodrug is to be converted to an anti-tumour agent under the influence of an enzyme that is linkable to a monoclonal antibody that will bind to a tumour associated antigen, the combination of such a prodrug with such an enzyme monoclonal/antibody conjugate represents a very powerful clinical agent.

We have now discovered new nitroreductases, obtainable from bacterial sources, that are of interest in that not only are they capable of converting CB 1954 into an active anti-tumour agent, but also, unlike EC.1.6.99.2, capable of converting CB 1954 analogues which are also prodrugs into active anti-tumour agents.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the results of an experiment in which CB 1954 (100 μM) and reduced cofactor (500 μM) were incubated with enzyme (2 mg/ml) E. coli nitroreductase (A) or 25 μg/ml Walker DT diaphorase (B) in 10 mM sodium phosphate buffer (pH7) in air at 37° C. At various times aliquots (10 μl) were injected onto a Partisil SCX (240×4.7 mm) HPLC column and eluted isocratically (2 ml/min) with 100 mM NaH₂ PO₄. The eluate was continuously monitored for absorption at 320, 260 and 360 nm and the concentration of CB 1954 calculated by integration of the peak corresponding to this compound on the HPLC trace.

FIG. 2 shows the formation of actinomycin D (AMD) during incubation of an AMD prodrug with a nitroreductase of the present invention.

FIG. 3 shows the formation of mitomycin C (MC) during incubation of an MC prodrug with a nitroreductase of the present invention.

FIG. 4 shows the binding in vitro of an antibody-enzyme conjugate according to the invention to cells.

The present invention provides a nitroreductase, obtainable from a bacterium having the following characteristics as exemplified by examples isolated from Escherichia coli B and Bacillus amyloliguifaciens:

1. It is a flavoprotein having a molecular weight in the range 20-60 Kilodaltons;

2. It requires either NADH or NAD(P)H or analogues thereof as a cofactor.

3. It has a Km for NADH or NAD(P)H in the range 1-100 μM.

4. It is capable of reducing either or both nitro groups of CB 1954 and analogues thereof to a cytotoxic form e.g. the hydroxylamine.

The nitroreductases of the invention occur naturally within the cells of E. coli B , E. coli C and other E. coli strains e.g. K12 type as well as other gram negative organisms e.g. Thermus aguaticus, and gram positive bacteria such as Bacillus amyloliguifacieans and Bacillus caldotenax. They can be recovered from such cells by disrupting the cells and subjecting the cell contents to chromatographic separation and isolating the nitroreductase.

For example, the nitroreductase of the present invention from E. coli B has been purified to homogeneity--see Table 1 and has been subjected to amino acid sequence analysis with the results set out in Table 2. The upper sequence shows the deduced amino acid sequence of the 219-mer nitroreductase obtained from Salmonella typhimurium as described by Watanabe et al, Nucl. Acids, Res. 18, 1059 (1990). The lower sequence in bold type shows the sequence of the cyanogen bromide fragments of the E. coli B nitroreductase as an example of the present invention showing a certain degree of homogeneity but sufficient differences to confirm that it is nitroreductase that is different from that of Watanabe et al and the recently described Enterobacter cloacae nitroreductases, see Bryant et al, J. Biol Chem. 266, 4126 (1991) or the Walker cell nitroreductase and is a previously unreported enzyme.

The amino acid sequence of the E. coli B nitroreductase of the invention can also been derived from sequencing the nucleotides in the nitroreductase gene and these sequences are set out below in Table 3. The nucleotide sequence of Table 3 has been used to prepare the attached sequence listings.

Using the information in Table 3, a nitroreductase according to the present invention may be prepared by expressing DNA encoding the nitroreductase in a suitable expression vector contained in a host cell, and recovering the nitroreductase. The expression vector may be, for example, a bacterial, yeast, insect or mammalian expression vector and the host cell will be selected to be compatible with the vector.

As indicated above, the new enzymes of the present invention are capable of reducing a nitro group in various substrate molecules and we have found that the enzymes are particularly useful in their ability to reduce the nitro group of various p-nitrobenzyloxycarbonyl derivatives of cytotoxic compounds to give "self-immolative" compounds that automatically decompose to release cytotoxic compounds.

The interest in the present approach resides in the fact that the cytotoxicity of various cytotoxic compounds containing amino or hydroxy substituents, particularly aromatic amino or hydroxy substituents give rise to p-nitrobenzyloxycarbonyl derivatives of the amino or hydroxy group which exhibit considerably less cytotoxicity than the amino or hydroxy parent compound. Thus, it is possible to use the p-nitrobenzyloxycarbonyl derivatives as prodrugs in a system of the type discussed above where the prodrug is converted into an anti-tumour agent under the influence of an enzyme that is linkable to a monoclonal antibody that will bind to the tumour associated antigen.

Accordingly, the present invention provides new compounds of the general formula: ##STR4## and: ##STR5## where R¹ and R² are groups such that the compound R¹ NH₂ and R² OH are cytotoxic compounds.

It is preferred that compounds R¹ NH₂ and R² OH are aromatic cytotoxic compounds and the compounds R¹ NH₂ can be any one of the well known nitrogen mustard compounds, for example based on p-phenylene diamine. Thus, the compound R¹ NH₂ can be: ##STR6## or analogues of this compound with the general structure IV ##STR7## where R' and R" are H, F or CH₃, and particularly where R'=H and R"=CH₃ ;

or R'=CH₃ and R"=H;

or R'=H and R"=F;

or R'=F and R"=H.

A further type of amino cytotoxic compound that can be used in accordance with the present invention are compounds such as actinomycin D, doxorubicin, daunomycin and mitomycin C. The structure of the pro-drugs derived from actinomycin D, doxorubicin and mitomycin C are shown below as V, VI and VII respectively. ##STR8##

Similar p-nitrobenzyloxy derivatives can be made at the amino substituent of other actinomycins and of the other cytotoxic compounds of the type mentioned above.

In addition to forming p-nitrobenzyloxycarbonyl derivatives at an amino group on a cytotoxic compound, similar derivatives can be made at a hydroxy group, particularly a phenolic hydroxy group of a cytotoxic compound. Here, attention is directed at the phenolic nitrogen mustard compounds, and a specific compound of the invention of this type is of the formula: ##STR9##

The new enzymes of the present invention are capable not only of reducing at least one of the nitro groups of CB 1954 but also at least one of the nitro groups of certain CB 1954 analogues, e.g. descarboxamido CB1954 (1-aziridin-1-yl-2,4-dinitrobenzamide--known as CB 1837) and N,N-dimethyl CB 1954 [N,N-dimethyl-(5-aziridin-1-yl-2,4-dinitrobenzamide also known as CB 10-107]. The new enzymes of the present invention are also capable of reducing the nitro groups of other aromatic nitro compounds such as 5-chloro-2,4-dinitrobenzamide, 3,5-dinitrobenzamide, 3-nitrobenzamide, 4-nitrobenzamide and 5-nitro-2-furaldehydesemicarbazone (nitrofurazone).

The present invention also provides hydroxylamino derivatives of the general formula X: ##STR10## wherein X¹ and X², which may be the same or different, are each NHOR⁵ or NO₂ with the proviso that X¹ and X² are not both NO₂, where R⁵ is H or a carboxylic acyl or hydrocarbyl group as defined above in relation to formula IX and Y is H or CON(CH₃)₂. In the compounds of formula X containing two hydroxylamino groups, the group R can be the same or different but will normally be the same.

As mentioned above, the new p-nitrophenylbenzyloxy compounds of the invention of formulae I and II are of interest in that they have a reduced cytotoxicity compared to that of the cytotoxic compound R¹ NH₂ or R² OH from which they are derived and they are capable of acting as a substrate for the nitroreductase of the present invention. While the present invention is not dependent, for its definition, upon the exact mode of action of the nitroreductase on the compound of formula I or II, it is believed that the nitro group of the p-nitrophenyl-benzyloxycarbonyl residue is converted to the corresponding amino or hydroxylamino group and that the resulting p-aminobenzyloxycarbonyl or p-hydroxyl-aminobenzyloxycarbonyl compound automatically degrades under the reaction conditions used for the enzymatic reduction to release the cytotoxic compound and form p-aminobenzyl alcohol or p-hydroxylaminobenzyl alcohol and carbon dioxide as by products in accordance with the following reaction scheme: ##STR11##

The p-nitrobenzyloxycarbonyl compounds of the invention are conveniently prepared by methods of chemical synthesis known per se. For example, the amine or hydroxy cytotoxic compounds can be reacted with 4-nitrobenzyl chloroformate under anhydrous conditions in the presence of a hydrogen chloride aceptor, particularly an alkylamine such as triethylamine. This reaction can be carried out in a dry organic solvent such as chloroform and the resulting compound of the invention of formula I or formula II isolated from the organic solvent by conventional methods such as chromatography.

The new compounds of the present invention IX and X will normally be prepared by subjecting the corresponding dinitro compound to the action of the new nitroreductase of the present invention. It is, of course, also possible to produce the new compounds of the present invention IX and X by chemical synthesis, using selective reducing agents followed by conversion of the resulting hydroxylamine to its corresponding ester or ether. The esters and ethers are more conveniently prepared semi-synthetically by carrying out the reduction of the nitro group with the nitroreductase of the present invention to give the corresponding hydroxylamine that is then converted by known chemical methods to the corresponding ester or ether.

One of the most convenient ways of producing the new nitroreductase of the invention is to recover the material from the cell contents of bacteria such as E. coli B. Alternatively, it is possible to clone the gene, isolatable from the bacteria, encoding the desired enzyme and to transfer the cloned gene, in a suitable vector system, into another host cell from which or in which it can be expressed.

In order to bring about the enzymatic reduction of CB 1954 and its analogues with the new enzymes of the present invention, it is necessary to have a cofactor present in the reaction system. The ability of the enzyme of the present invention to bring about this reduction can be demonstrated experimentally by the use of NADH or NAD(P)H as the cofactor but the use of such cofactors in clinical practice may be problematic in view of the ease with which NAD(P)H particularly is oxidised by other enzymes present in the body and the lack of selectivity of the cofactors between the various mammalian and non-mammalian enzymes. We have now found that the riboside of 1,4-dihydro-nicotinic acid is as least as effective as a cofactor in the nitroreductase reduction of CB 1954 and analogues thereof and moreover, because of its selectivity to the E. coli nitroreductase of the invention, is more suited to clinical use which makes its incorporation in a multi-component system of the type described below particularly valuable.

The riboside of 1,4-dihydro-nicotinic acid which can be used in the present invention as a cofactor is a new compound and forms part of the present invention. It can be prepared from commercially available nicotinic acid ribotide which is first converted to the corresponding riboside by enzymatic dephosphorylation e.g. using an alkaline phosphatase. The riboside, obtained by such enzymatic dephosphorylation, or by chemical synthesis, using the method described by Jarman, J. Chem. Soc. (c) 918-920 (1969) can then be reduced, e.g. using an alkali metal hydrosulphite, to give 1,4-dihydro-nicotinic acid riboside.

One of the most important practical applications of the new enzymes of the present invention is that they can be used in association with nitro compounds that are prodrugs for anti-tumour agents and so provide a system of cancer chemotherapy where the extent of exposure of the patient to the cytotoxic agent is limited, so far as possible, to those regions where there is the interaction between the prodrug and the nitroreductase of the invention. Thus, one aspect of the present invention is to provide a method of chemotherapy and a system for chemotherapy involving the conjoint use of the nitroreductase of the present invention in association with a nitro compound which is a prodrug for a cytotoxic compound.

The most or one of the most convenient ways of utilising the system of the present invention is to conjugate the nitroreductase of the present invention to a targeting agent such as a monoclonal antibody that will bind with a tumour-associated antigen.

As used herein, the term "monoclonal antibody" will be understood by those of skill in the art not simply to refer to antibodies produced by traditional hybridoma techniques, but also to cover antibodies and variants thereof produced by recombinant means. These include, for example, humanised antibodies such as those with a constant region from a human antibody grafted onto a non-human antibody variable region (see for example EP-A-O 120 694), chimeric antibodies such as those with non-human complementarity determining regions (CDRs) grafted into a human variable region framework (see for example EP-A-0 239 400) and single chain antibodies. Fragments of such monoclonal antibodies which retain their target binding activity are also included by the general term "monoclonal antibody". This includes Fab' and F(ab')₂ fragments.

The selection of monoclonal antibody will clearly be influenced by the nature of the target tumour but for the purposes of illustrating the present invention, reference may be made to the anti-CEA antibody A₅ B₇.

As an alternative to the use of a monoclonal antibody, it is also envisaged that other targeting agents to which the nitroreductase of the present invention is conjugated may be used. For example, it is known that certain soluble macromolecules can be used for passive tumour targeting of certain tumour types. Many solid tumours possess vasculature that is hyperpermeable to macromolecules. Although the reasons for this are not clearly understood, the result is that such tumours can selectively accumulate circulating macromolecules. The enhanced permeability and retention effect (EPR effect) is thought to constitute the mechanism of action of SMANCS (styrene/maleic-anhydride-neocarzinostatin), now in regular clinical use in Japan for the treatment of hepatoma. Another class of conjugates under investigation for anticancer activity is N-(2-hydroxypropyl)methacrylamide copolymer-anthracycline conjugates (L. Seymour, Critical Reviews in Therapeutic Drug Carrier Systems, 9(2) 135-187 (1992)). Thus, conjugates of a polymer, including styrene/maleic-anhydride or N-(2-hydroxypropyl)methacrylamide copolymer, and the nitroreductase of the invention can be used place of conjugates of a monoclonal antibody and enzyme.

With this system, it is possible in a course of cancer chemotherapy to administer to the patient requiring the treatment the nitro compound which is the prodrug for the cytotoxic compound and the enzyme/targeting agent conjugate. The prodrug and the conjugate can be administered simultaneously but it is often found preferable, in clinical practice, to administer the enzyme/agent conjugate before the prodrug, e.g. up to 72 hours before, in order to give the enzyme/agent conjugate an opportunity to localise in the region of the tumour target. By operating in this way, when the prodrug is administered, conversion of the prodrug to the cytotoxic agent tends to be confined to the regions where the enzyme/agent conjugate is localised, i.e. the region of the target tumour and damage to healthy cells caused by the premature release of the cytotoxic agent is minimised.

The degree of localisation of the enzyme/agent conjugate (in terms of the ratio of localized to freely circulating active conjugate) can be further enhanced using the clearance and/or inactivation systems described in WO89/10140. This involves, usually following administration of the conjugate and before administration of the prodrug, the administration of a component (a "second component") which is able to bind to the such part of the conjugate so as to inactivate the enzyme and/or accelerate the clearance of the conjugate from the blood. Such a component may include an antibody to the nitroreductase of the invention which is capable of inactivating the enzyme.

The second component may be linked to a macromolecule such as dextran, a liposome, albumin, macroglobulin or a blood group O erythrocyte so that the second component is restrained from leaving the vascular compartment. In addition or as an alternative, the second component may include a sufficient number of covalently bound galactose residues, or residues of other sugars such as lactose or mannose, so that it can bind the conjugate in plasma but be removed together with the conjugate from plasma by receptors for galactose or other sugars in the liver. The second component should be administered and designed for use such that it will not, to any appreciable extent, enter the extravascular space of the tumour where it could inactivate localised conjugate prior to and during administration of the prodrug.

The exact dosage regime will, of course, need to be determined by individual clinicians for individual patients and this, in turn, will be controlled by the exact nature of the prodrug and the cytotoxic agent to be released from the prodrug but some general guidance can be given. Chemotherapy of this type will normally involve parenteral administration of both the prodrug and the enzyme/agent conjugate and administration by the intravenous route is frequently found to be the most practical.

Bearing in mind the manner in which the enzymes of the present invention are to be used, the present invention extends to pharmaceutical compositions comprising the enzyme, preferably conjugated to a targeting agent such as a monoclonal antibody capable of binding to a tumour-associated antigen in association with a pharmaceutically acceptable carrier or diluent, normally one suitable for parenteral administration.

The present invention also extends to pharmaceutical compositions comprising one or more of the p-nitrobenzyloxycarbonyl compounds of the present invention of formula I, formula II, formula V, formula VI or formula VII in association with a pharmaceutically acceptable carrier or diluent, normally one for parenteral administration.

The present invention further extends to pharmaceutical compositions comprising the hydroxylamino anti-tumour agents of the present invention of formula IX or formula X in association with a pharmaceutically acceptable carrier or diluent, normally one for parenteral administration.

The present invention also provides a system for use in the control of neoplasia in a human or animal subject comprising the nitroreductases of the present invention, preferably conjugated with a targeting agent such as monoclonal antibody that will bind to a tumour-associated antigen, in association with at least one of a p-nitrobenzyloxycarbonyl compound of Formula I, II, V, VI or VII which is a prodrug for an anti-tumour agent and preferably a riboside or ribotide of nicotinic acid or nicotinamide to act as a cofactor for the enzyme. The present invention extends to a method of treating neoplasia in a human or animal host requring such treatment which comprises administering to the host an effective amount of a p-nitrobenzyloxycarbonyl compound of Formula I, II, V, VI or VII which is a prodrug for an anti-tumour agent and the enzyme of the present invention, preferably conjugated with a targeting agent such as a monoclonal antibody that will bind to a tumour-associated antigen, the enzyme preferably being used in association with an ribotide or riboside of nicotinic acid or nicotinamide as cofactor for the enzyme.

The present invention further provides a system for use in the control of neoplasia in a human or animal subject comprising a nitroreductase of the present invention, preferably conjugated with a targeting agent such as a monoclonal antibody that will bind to a tumour-associated antigen, in association with a nitro compound which is a prodrug for an anti-tumour agents of the formula IX or X and preferably a riboside or ribotide of nicotinic acid or nicotinamide to act as a cofactor for the enzyme. The present invention also provides a method of treating neoplasia in a human or animal host requiring such treatment which comprises administering to the host an effective amount of a nitro compound which is a prodrug for an anti-tumour agents of the formula IX or X and the enzyme of the present invention, preferably conjugated with a targeting agent such as a monoclonal antibody that will bind to a tumour-associated antigen, the enzyme preferably being used in association with an ribotide or riboside of nicotinic acid or nicotinamide as cofactor for the enzyme.

The various systems for use in the treatment of neoplasia described above optionally include the "second component" for accelerated clearance described above. Likewise, the methods of treatment of neoplasia described above optionally include as part of that method the use of the second component, an effective amount of which is administered after administration of the enzyme, in order to increase the ratio of localised to freely circulating enzyme. Reference may be made to WO89/10140 for further particular details of the second component, and such details can be incorportated for use in the present invention.

The present invention is further illustrated by the following Examples.

EXAMPLE I Isolation and Purification of a Nitroreductase Enzyme from E. coli B.

200 grams of E. coli B cell paste were resuspended to a total volume of 1 litre of 20 mM potassium phosphate buffer, pH 7, containing 0.3M ammonium sulphate. The cells were broken ultrasonically using an MSE Soniprep 150 disintegrator (3×30 seconds on full power with 60 second intervals to allow heat to dissipate). To aid clarification of the extract, DNase (23,000 Kunitz units/L) and RNase (2,400 Kunitz units/L) were added prior to centrifugation at 8,000 g for 30 minutes to remove cell debris. The clear yellowish supernatant was passed through a 0.45 μm filter prior to chromatography.

The filtered extract was applied to a column (25×5 cm) of Phenyl-Sepharose CL-6B (Pharmacia) in 20 mM potassium phosphate buffer, pH 7, containing 0.3M ammonium sulphate. After washing with 2 column volumes of starting buffer, the column was eluted with 10 mM Tris-HCl buffer, pH 7.6. Active fractions were pooled and dialysed for 18 hours against 2 0 mM Tris-HCl, pH 7.6, to remove traces of ammonium sulphate. The dialysed fractions were applied in 50 ml aliquots to Q-Sepharose-High Performance in 20 mM Tris-HCl, pH 7.6, at a flow rate of 4 ml per minute. Elution was by a 0-0.2M gradient of KCl, the nitroreductase eluting at 0.1-0.12M KCl . Active fractions were pooled and desalted into 20 mM Bis Tris propane, pH 7, using a column (32×6 cm) of Sephadex G25 medium. These fractions were applied to Q-Sepharose High Performance (Hi-Load 26/10 column, Pharmacia) equilibrated in 20 mM Bis Tris propane, pH7. Elution was by a 0-0.1M gradient of KCl. Nitroreductase eluted as the first major peak at 0.07-0.09M KCl.

Homogeneity of the final product was ascertained using precast 8-25% gradient gels for native polyacrylamide gel electrophoresis (Pharmacia Phastsystem). Electrophoresis was performed for 75 vh.

The nitroreductase in crude and partially purified fractions was routinely assayed by its quinone reductase activity using menadione as substrate, NADH as co-factor and cytochrome C as terminal electron acceptor.

Determination of Isoelectric Point

The isoelectric point of nitroreductase was determined by isoelectric focusing (Pharmacia Phastsystem, focusing for 400 vh) and chromatofocusing using a Mono P column (Pharmacia Mono P HR5/20, 20 mM Bis-Tris pH 6.3 and polybuffer 74, pH 4.0).

The E. coli nitroreductase was isolated as a pure protein with a molecular weight of 24 kDa (as determined by both SDS-polyacrylamide gel electrophoresis and gel filtration chromatography). A second protein, which had quinone reductase activity but was inactive as a nitroreductase against CB 1954, partially co-elutes from Phenyl Sepharose and can be fully separated from the active enzyme by the ion exchange chromatography step on Q-Sepharose high performance (see Table 1) at pH 7.6. The two enzymes differ in molecular weight (inactive 55 KDa; active 24 KDa) and isoelectric point (inactive 5.2; active 5.1). The active protein has a yellow coloration suggesting the presence of a flavin coenzyme. After heating at 70° C. for 20 minutes this flavin could be separated from the apoenzyme by ultrafiltration and shown to be FMN, rather than FAD, using HPLC.

                                      TABLE 1                                      __________________________________________________________________________     The purification of a nitroreductase from E. coli B. The enzyme activity       was assayed, at 37° C.                                                    by its quinone reductase activity using menadione (10 μM) as              substrate, NADH (500 μM) as                                                   cofactor and cytochrome C (70 μM) as terminal electron acceptor. A        unit was defined as 1 μmole                                                   of cytochrome C reduced per minute.                                                  TOTAL ACTIVITY                                                                            SPECIFIC ACTIVITY                                                                        YIELD                                               FRACTION (Units) (Units/mg protein) (%)                                      __________________________________________________________________________     Crude   3784*      0.34      100                                                 Phenyl Sepharose 2371* 1.6  63                                                       CB 1954                                                                             CB 1954         CB 1954                                                                             CB 1954                                         ACTIVE INACTIVE  ACTIVE INACTIVE                                               1109 1262  30 33                                                              Q-Sepharose 666 -- 79 18 --                                                    (Tris, pH 7.6)                                                                 Q-Sepharose 310 -- 130  8 --                                                   (Bis-Tris propane,                                                             PH 7)                                                                        __________________________________________________________________________      *This includes activity form enzymes not active against CB 1954          

Fragments suitable for sequence analysis were produced by digestion of the enzyme with cyanogen bromide. The peptides which resulted from these digests were purified by reverse-phase HPLC, using a RP 300 column (25×4.6 mm) (Brownlee) and a solvent gradient of 10-60% acetonitrile in water with 0.06% trifluoroacetic aid in each solvent. Sequence analysis was performed by automated Edman degradation using an Applied Biosystems 470A gas phase protein sequencer (Kelvin Close, Warrington, U.K.).

Amino acid sequence analysis of the nitroreductase

The E. coli nitroreductase as isolated above was subject to amino acid sequence analysis. In contrast to the enzyme isolated from the Walker tumour, EC.1.6.99.2, the nitroreductase of the present invention was not blocked at the N-terminus and gave a clear N-terminal sequence of 31 amino acid (SEQ ID NO:4) residues and a peptide, generated by digestion with cyanogen bromide, of a further 41 residues (SEQ ID NO:5). These partial sequences are given in Table 2.

                                      TABLE 2                                      __________________________________________________________________________     Amino acid sequences of the N-terminus of the E. coli B nitroreductase          (SEQ ID NO.:4) and of a peptide (SEQ ID NO.:5) obtained after digestion        of the nitroreductase with cyanogen bromide (bold type) and a comparison       with the deduced protein sequence of the nitroreductase from                   Salmonella typhimurium (SEQ ID NO.:3) (Watanabe et al, 1990).                 __________________________________________________________________________     1                 10                  20                      30                M D I V S V A L Q R Y S T K A F D P S K K L T  A  E E A  D  K I K T L          M D I I S V A L K R H S T K A F D A S K - L T (P) E Q A (D) Q I K               -             40                  50                  60                      L Q Y S P S S T N S Q P W H F I V A S T E E G K A R V A K S A A G N Y           -   70                  80                  90                  100           T F N E R K M L D A S H V V V F C A K T A N D D A W L E R V V D Q E D           -             110                 120                 130                     A D G R F A T P E A K A A N D K G R R F F A D M H R V S L K D D H Q W           -     140                 150             160                 170             M   A K V V Y L N V G N F L L G V A A M G L D A V P I E G F D A E V L          (M) A K Q V Y L N V G N F L L G V A A L G L D A V P I E G F D A A I L           -           180                 190                 200                       D A E F G L K E K G Y T S L V V V P V G H H S V E D F N A G L P K S R          D A E F G L K I                                                                 - 210               219                                                       L P L E T T L T E V                                                           __________________________________________________________________________

Nucleotide sequence of the nitroreductase gene

The nitroreductase gene of E. coli has been cloned and its nucleotide sequence determined by dideoxy sequence analysis of both strands. In Table 3 is shown the nucleotide sequence of 1167 base NruI/Pst fragment which contains an open reading frame of 651 nucleotides encoding the nitroreductase (SEQ ID NO:1). Putative sequence of Shine-Delgarno and transcriptional termination signal are indicated.

                                      TABLE 3                                      __________________________________________________________________________      NruI                                                                                                                TCGCGATCTGATCAACGATTCGTGGAATCTGGTGGT                                          TGATGGTCTGGCTAAACGCGATCA   60                                                    - AAAAAGAGTGCGTCCAGGCTAAAGCGGAAATCT                                          ATAGCGCATTTTTCTCGCTTACCATTT  120                                                 -                                                                                         S.D.                           CTCGTTGAACCTTGTAATCTGCTGGACGCAAAATTACTTTCACATGGAGT CTTTATGGA  180                                                                    m  d                      - TATCATTTCTGTCGCCTTAAAGCGTCATTCCACTAAGGCATTTGATGCCAGCAAAAAACT  240                                                 i  i  s  v  a  l  k  r  h  s  t                                             k  a  f  d  a  s  k  k  l                    - TACCCCGGAACAGGCCGAGCAGATCAAAACGCTACTGCAATACAGCCCATCCAGCACCAA  300                                                 t  p  e  q  a  e  q  i  k  t  l                                             l  g  y  s  p  s  s  t  n                    - CTCCCAGCCGTGGCATTTTATTGTTGCCAGCACGGAAGAAGGTAAAGCGCGTGTTGCCAA  360                                                 s  q  p  w  h  f  i  v  a  s  t                                             e  e  g  k  a  r  v  a  k                    - ATCCGCTGCCGGTAATTACGTGTTCAACGAGCGTAAAATGCTTGATGCCTCGCACGTCGT  420                                                 s  a  a  g  n  y  v  f  n  e  r                                             k  m  l  d  a  s  h  v  v                    - GGTGTTCTGTGCAAAAACCGCGATGGACGATGTCTGGCTGAAGCTGGTTGTTGACCAGGA  480                                                 v  f  c  a  k  t  a  m  d  d  v                                             w  l  k  l  v  v  d  q  e                    - AGATGCCGATGGCCGCTTTGCCACGCCGGAAGCGAAAGCCGCGAACGATAAAGGTCGCAA  540                                                 d  a  d  g  r  f  a  t  p  e  a                                             k  a  a  n  d  k  g  r  k                    -                          BglII                                              GTTCTTCGCTGATATGCACCGTAAAGATCTGCATGATGATGCAGAGTGGATGGCAAAACA  600                                                    f  f  a  d  m  h  r  k  d  l  h                                             d  d  a  e  w  m  a  k  q                    - GGTTTATCTCAACGTCGGTAACTTCCTGCTCGGCGTGGCGGCTCTGGGTCTGGACGCGGT  660                                                 v  y  l  n  v  g  n  f  l  l  g                                             v  a  a  l  g  l  d  a  v                    - ACCCATCGAAGGTTTTGACGCCGCCATCCTCGATGCAGAATTTGGTCTGAAAGAGAAAGG  720                                                 p  i  e  g  f  d  a  a  i  l  d                                             a  e  f  g  l  k  e  k  g                    - CTACACCAGTCTGGTGGTTGTTCCGGTAGGTCATCACAGCGTTGAAGATTTTAACGCTAC  780                                                 y  t  s  l  v  v  v  p  v  g  h                                             h  s  v  e  d  f  n  a  t                    - GCTGCCGAAATCTCGTCTGCCGCAAAACATCACCTTAACCGAAGTGTAATTCTCTCTTGC   840                                                l  p  k  s  r  l  p  q  n  i  t                                             l  t  e  v  .                                -     terminator                                                              CGGGCATCTGCCCGGCTATTTCCTCTCAGATTCTCCTGATTTGCATAACCCTGTTTCAGC  900                                                   - CGTCATCATAGGCTGCTGTTGTATAAAGGAGAC                                          GTTATGCAGGATTTAATATCCCAGGTT  960                                                 - GAAGATTTAGCGGGTATTGAGATCGATCACACC                                          ACCTCGATGGTGATGATTTTCGGTATT 1020                                                 - ATTTTTCTGACCGCCGTCGTGGTGCATATTATT                                          TTGCATTGGGTGGTACTGCGGACCTTC 1080                                                 - GAAAAACGTGCCATCGCCAGTTCACGGCTTTGG                                          TTGCAAATCATTACCCAGAATAAACTC 1140                                                 -                       PstI                                                  TTCCACCGTTTAGCTTTTACCCTGCAG              __________________________________________________________________________                                          1167                                 

EXAMPLE 2

Enzymatic reduction of CB 1954. CB 1954 (100 μM and also containing [U-³ H] C.B 1954 at 1.6×10⁵ dpm per nmole), NADH or NAD(P)H (500 μM) were incubated with the active enzyme obtained as described in Example 1, (generally 2 μg/ml E. coli nitroreductase or 35 μg/ml Walker NAD(P)H dehydrogenase (quinone) EC 1.6.99.2) in 10 mM sodium phosphate buffer (pH 7) under either air or helium. At various times aliquots (10 μl) were injected onto a Partisphere SCX (110×4.7 mm) HPLC column and eluted isocratically (2 ml/min) with 100 mM NaH₂ PO₄.

The eluate was continuously monitored for adsorption at 310, 260 and 360 nm and the spectra of eluting components recorded using a diode-array detector. Samples (0.5 ml) were collected and the tritium activity of each determined by liquid scintillation counting. This separation system could resolve all the expected reduction products.

To confirm further the identity of any reduction products, the above reduction mixture was also injected onto an ODS-5 reverse phase HPLC column and eluted (1 ml/min) with a methanol gradient (0-30% linear over 30 min, 30-100% linear over 10 min.) in 0.1 M sodium phosphate buffer (pH 7).

Reduction of CB 1954 by the E. coli nitroreductase resulted in the formation of two products, shown to be, by comparison of retention times and spectral characteristics with known standards, 5-(aziridin-1-yl)-4-hydroxylamino-2-nitrobenzamide and 5-(aziridin-1-yl)-2-hydroxylamino-4-nitrobenzamide. No other CB 1954 metabolites were found. In further confirmation of the formation of both the 2 and the 4-hydroxylamines by the nitroreductase of the invention, 33 μM of 4-hydroxylamine was formed when 67 μM of CB 1954 was reduced by the nitroreductase. In contrast 50 μM of 4-hydroxylamine was formed by the reduction of 50 μM of CB 1954 by the Walker NAD(P)H dehydrogenase (quinone). Based on initial rates of 4-hydroxylamine formation nitroreductase is 31.2 fold more active per mg protein than NAD(P)H dehydrogenase (quinone) (or 62 fold more active by CB 1954 reduction) under the standard conditions used.

The rate of reduction of CB 1954 or product formation was the same when the co-factor was either NAD(P)H or NADH and when the reduction was performed under helium.

To show that the nitroreductase was producing a cytotoxic species, the reduction of CB 1954 was carried out in the presence of V79 cells, which are insensitive to CB 1954. As shown in Table 4, a dramatic cytotoxic effect was observed in Hamster V79 cells--but only under those conditions in which the nitroreductase reduced CB 1954.

                  TABLE 4                                                          ______________________________________                                         The effect of CB 1954 on the survival of V79 cells in the presence of the       E. coli B nitroreductase. All treatments were for 2 hours at 37°       C. and the                                                                       cells were then plated out for their resulting colony-forming ability.       The                                                                              nitroreductase concentration was 2 μg/ml and NADH was used as a            cofactor.                                                                                                      % DRUG                                                                          TREATMENT % SURVIVAL REDUCTION               ______________________________________                                         CONTROL           100        --                                                  +500 μM NADH 100 --                                                         +50 μM CB 1954 100 <1.0                                                     +NADH + CB 195 41 <1.0                                                         +Nitroreductase (NR) 94 --                                                     +NR + 50 μM CB 1954 99 <1.0                                                 +NR + CB 1954 + 500 μM NADH 0.024 72                                      ______________________________________                                    

EXAMPLE 3 Substrate Specificity of the E. coli Nitroreductase Enzyme

The ability of the E. coli nitroreductase of the invention to reduce nitro-compounds other than CB 1954 was determined by HPLC by following the decrease in the peak area of NADH resulting from its oxidation. The experiments were carried out as above but the aliquots were injected onto a Partisphere SAX (110×4.7 mm) HPLC column and eluted isocratically (1 ml/min) with 75 mM NaH₂ PO₄. The results are shown in Table 5.

                  TABLE 5                                                          ______________________________________                                         The relative rates of reduction of various nitrobenzamides and                   nitrobenzenes with E. coli nitroreductase enzyme. Reduction                    rates were determined by the resulting oxidation of NADH. All                  reactions were carried out at 37° C. in air, with NADH (500           μM) as                                                                        electron donor, at an initial substrate concentration of 100 μM.                                  RELATIVE RATE                                                                    OF NADH OXIDATION                                      SUBSTRATE NR                                                                 ______________________________________                                         CB 1954             1.0                                                          2,4-dinitro-5-(2-hydroxy- 0.04                                                 ethylamino)benzamide                                                           2-amino-5-(aziridin-1-yl)- <0.01                                               4-nitrobenzamide                                                               4-amino-5-(aziridin-1-yl)- <0.01                                               4-nitrobenzamide                                                               5-chloro-2,4-dinitrobenzamide 22.4                                             3,5-dinitrobenzamide 75.5                                                      2-nitrobenzamide 0.06                                                          3-nitrobenzamide 1.8                                                           4-nitrobenzamide 5.1                                                           2,4-dinitrophenol <0.01                                                        5-nitro-2-furaldehydesemicarbazone 3.6                                         (nitrofurazone)                                                              ______________________________________                                    

EXAMPLE 4

Enzyme Kinetic and Inhibition Studies

Quinone reductase activities were assayed by a spectrophotometric method using menadione as a substrate and cytochrome c as a terminal electron acceptor as described in Knox et al, Biochem. Pharmacol., 37, 4671-4677, 1988. Initial rates of reaction were determined by linear regression analysis (r>0.995) and kinetic parameters determined from the resulting plots as described by Roberts et al, Biochem. Pharmacol. 38, 4137-4143, 1989. Protein concentration was determined using the a conventional protein assay (Bio-Rad) calibrated against bovine serum albumin.

Kinetic parameters for the E. coli nitroreductase and Walker NAD(P)H, dehydrogenase (quinone) EC. 1.6.99.2, are given in Table 6. Although both enzymes have comparable Km's for CB 1954, the Km of the nitroreductase for NADH is about 10 fold less than the Walker enzyme. The absolute rates of reduction of CB 1954 (i.e. under saturating conditions) by the two enzymes is their k_(cat) values and this is 90 fold higher for the E. coli nitroreductase. Menadione was also a substrate for both enzymes with little difference in their respective k_(cat) 's although the Km of nitroreductase for this substrate was 60 fold higher.

Dicoumarol was an inhibitor of the nitroreductase. No kinetic parameters could be measured with respect to NADH. With respect to menadione, dicoumarol was an uncompetitive inhibitor with a Ki' of 1.90±0.38 μM.

                  TABLE 6                                                          ______________________________________                                         Kinetic parameters for the E. coli B nitroreductase and Walker                   NAD(P)H dehydrogenase (quinone).                                                 COMPOUND            NR        WALKER                                       ______________________________________                                         NADH        Km      -6 μM    75 μM                                         CB 1954 Km 862 ± 145 μM 826 ± 46 μM                                 K.sub.cat 360 min.sup.-1 4 min.sup.-1                                         MENADIONE km 80 μM 1.3 μM                                                 K.sub.cat 4.2 × 10.sup.4 min.sup.-1 6.5 × 10.sup.4 min -        ______________________________________                                                                         1                                         

EXAMPLE 5

The Ability of the E. coli B Nitroreductase to Activate Compounds other than CB 1954 to a Cytotoxic Species.

As shown in Table 7, a large cytotoxic effect was observed in human MAWI cells by the prodrugs CB 1837 and CB 10-107 but only under those conditions when the prodrug was reduced by the nitroreductase enzyme.

                  TABLE 7                                                          ______________________________________                                         The effect of enzyme-activated prodrugs on the survival of MAWI                  cells. 1 ml volumes of MAWI cells (2 × 10.sup.5 /ml) were              incubated                                                                        with 50 μM prodrug, 500 μM NADH, and 10 μg/ml the enzyme of          Example 1. After a 2 hour incubation at 37° C., the cells were          harvested and assayed for their colony forming ability, and the                supernatant assayed for the concentration of remaining prodrug                 by HPLC.                                                                        TREATMENT        % SURVIVAL % DRUG REDUCTION                                 ______________________________________                                         CONTROL        100        --                                                     + 500 μM NADH 100 --                                                      + 50 μM CB 1837                                                                            97.4       <1.0                                                   + NADH + CB 1837 97.4 <1.0                                                     + 50 μM CB 10-107 85.6 <1.0                                                 + NADH + CB 10-107 87.9 <1.0                                                   + NR + CB 1837 + 1.56 >95                                                      500 μM NADH                                                                 + NR + CB 10-107 + NADH 5.0 45                                               ______________________________________                                    

EXAMPLE 6 PREPARATION OF 1,4-DIHYDRO-NICOTINIC ACID RIBOSIDE FROM EITHER NICOTINIC ACID RIBOSIDE OR NICOTINIC ACID RIBOTIDE

(i) Preparation of Nicotinic Acid Riboside from Nicotinic Acid Ribotide.

A solution of nicotinic acid ribotide (nicotinic acid mononucleotide) (Sigma, Poole, U.K.) (25 mg) in aqueous buffer (tris 100 mM; pH 8.5; MgCl₂ ; 2.5 ml), was treated with 2000 units of alkaline phosphatase, type VII-S (100 μl; 20,000 units/ml) (Sigma) at 37° C. for 1 hour. The alkaline phosphatase was separated from the digest by centrifugal molecular filtration (10,000 molecular weight limit; "Centricon 10"; Amicon, High Wycombe, U.K.). Dilutions (1:100) of the solution were analysed both before and after digestion with alkaline phosphatase by anion exchange high performance liquid chromatography (Partisphere 5-SAX column, eluted isocratically with 0.1M NaH₂ PO₄, pH5, 1.5 ml/min, 10 μl injection volume, monitored by UV absorbance at 260 nm). The parent compound eluted with a retention time of 1.18 minutes. Post digestion examination indicated that a complete conversion had occurred to give the novel title compound eluting with a retention time of 0.63 minutes, taken to be indicative of dephosphorylation, yielding the riboside.

(ii) Reduction of Nicotinic Acid Riboside to 1,4-dihydro-nicotinic Acid Riboside.

The method of Jarman and Searle (1972) was adopted. The method was applied both to nicotinic acid riboside as produced above, and also to chemically synthesised material (Jarman 1969). Identical results were obtained with starting compound from either source of origin.

To an aqueous solution of nicotinic acid riboside (5 ml; 4 mg/ml) was added 50 mg sodium carbonate, 50 mg sodium bicarbonate, and 50 mg sodium hydrosulphite. The stoppered solution was incubated at 37° C. for 1 hour and the reduction product separated by preparative reverse phase HPLC. The 5 ml was injected onto a microsorb 5 μm C18 (10×250 mm) reverse-phase column (Rainin) and eluted by a gradient of methanol in water (0-100% over 30 minutes) buffered at pH 5 by 10 mM NH₄ CH₃ COO at 4 ml/min. The eluate was continuously monitored both by UV absorbance and by fluorescence and the reduction product, (chromatographing with baseline resolution at a retention time of 12-13 minutes), was characterised by both its absorbance maximum at 326 nm (at pH5; 340 nm at pH7) and by its fluorescence (Gilson 121 fluorometer with wide band glass filters; excitation centred at 350 nm; emission at 450 nm), neither of which properties are displayed by the parent compound. Eluates of 4 ml. of a 2 mM solution (assuming an ε₃₄₀ of 6200 (i.e. the same as NADH) were typical, indicating a yield of 2.7 mg i.e. approximately 13%. Prior to experimental usage, the preparations were analysed by analytical HPLC and confirmed to be essentially pure.

References:

(i) M. Jarman and F. Searle, Potential Coenzyme Inhibitors-V, Biochem. Pharmacol. Vol 21, pp. 455-464, 1972.

(ii) M. Jarman, 4-Substituted Nicotinic acids and Nicotinamides. Part III. Preparation of 4-Methylnicotinic acid Riboside. J. Chem. Soc. (C), 918-920, 1969.

EXAMPLE 7 The Ability of 1,4-dihydro-nicotinic Acid Riboside to Act as a Cofactor for the E. coli Nitroreductase.

The ability of the nitroreductase enzyme to be able to use a synthetic cofactor in place of NADH or NADPH for the reduction of CB 1954 is shown in FIG. 1. The nitroreductase enzyme can use both 1,4-dihydro-nicotinic acid riboside and NADH with equal efficiency as cofactors for the reduction of CB 1954. In contrast Walker DT diaphorase cannot utilise this synthetic cofactor. Therefore 1,4-dihydro-nicotinic acid riboside is a selective cofactor for the E. coli nitroreductase enzyme and cannot be used by mammalian DT diaphorase.

In the following Examples 8-12, "Silica gel" refers to Merck silica gel 60, 70-230 mesh and Proton NMR spectra were obtained at 300 MHz in CDCl₃. Temperatures are in ° C.

EXAMPLE 8 4-[Bis(2-chloroethyl)amino]phenylcarbamic acid 4'-nitrobenzyl ester

To a stirred solution of 4-nitrobenzyl chloroformate (295 mg) in dry CHCl₃ (5 ml), a solution of 4-[bis(2-chloroethyl)amino]aniline hydrochloride (367 mg and NEt₃ (377 μl) in dry CHCl₃ (5 ml) was added over 5 min. After 1 hr the solution was kept at 20° for 18 hr, then evaporated. The residue was chromatographed on a column of silica gel with CHCl₃ to give the title product as a yellow solid which recrystallized from benzene/petroleum ether as prisms, m.p. 111-112°. Yield, 361 mg (64%). Chemical ionization mass spectrum with CH₄ : ion at m/Z 412 (M+1, relative intensity 1.00) indicates M=411. C₁₈ H₁₉ N₂ O₄ Cl₂ requires M=411 (for Cl³⁵ isotope). NMR: δ3.57 (m, 4H, CH₂ Cl or CH₂ N), 3.69 (M, 4H, CH₂ Cl or CH₂ N), 5.28 (s, 2H, ArCH₂), 6.65 (d, 4H, ArH), 7.51 (d, 2H, ArH) and 8.23 (d, 2H, ArH).

EXAMPLE 9 4-[Bis(2-chloroethyl)amino]phenyl 4'-nitrobenzyl carbonate

A solution of 4-nitrobenzyl chloroformate (58 mg) in dry CHCl₃ (1.5 ml) was added to a stirred, ice-cooled solution of 4-[bis(2-chloroethyl)amino]phenol hydrochloride (72 mg) and NEt₃ (74 μl) in dry CHCl₃ (2 ml). After 18 hr at 21°, the solution was evaporated and the residue chromatographed on a column of silica gel with CHCl₃ /petroleum ether (3:2) to give the title compound which crystallized from ETOAc/petroleum ether as pale yellow prisms, m.p. 77-790 (yield, 102 mg). FAB MS: ion at m/z 413 indicates M=412 (C₁₈ H₁₈ N₂ O₅ Cl₂ requires M=412). NMR (CDCl₃): δ3.62 (m, NCH₂ or ClCH₂), 3.69 (m, NCH₂ or ClCH₂), 5.33 (s, ArCH₂), 6.64 (d, ArH), 7.05 (d, ArH), 7.59 (d, ArH) and 8.25 (d, ArH).

EXAMPLE 10 N-4-Nitrobenzyloxycarbonyl-actinomycin D

Actinomycin D (AMD, 41 mg) in MeOH (5 ml) was hydrogenated over 10% Pd/C for 2 hr, then evaporated in vacuo. The residue under N₂ was dissolved in a solution of 4-nitrobenzyl chloroformate (18 mg) in dry CHCl₃ (1.5 ml) and a solution of NEt₃ (10 μl) in CHCl₃ (1.5 ml) was then added. After stirring under N₂ for 24 hr, the catalyst was filtered off and the solution, after dilution with MeOH (200 ml), was aerated for 3 days. The solution was evaporated and the product was purified to remove AMD by semi-preparative HPLC twice on a 1 cm diameter column of reversed-phased C₁₈ -bonded silica with a 50-100% gradient of MeCN in H₂ O. The title compound was crystallised from ethyl acetate/petroleum ether as red prisms. Yield, 31 mg (66%). Electrospray mass spectrum: ions at m/z 1434.5 (M+H) and 1456.4 (M+Na) indicates M=1433.5. C₇₀ H₉₁ N₁₃ O₂₀ (lowest mass isotope) requires M=1433.65. NMR gave the same signals as AMD plus δ6.71 (d, 1H, ArCH₂), 7.09 (d, 1H, ArCH₂) and additional signals in the aromatic region.

EXAMPLE 11 N-4-nitrobenzyloxycarbonyl-doxorubicin VI

Doxorubicin hydrochloride (2.25 mg) was dissolved in dimethylformamide (DMF) (0.3 ml) containing triethylamine (NEt₃) (0.55 μl) and a solution of 4-nitrobenzyl-4'-nitrophenylcarbonate (1.4 ml) in DMF (0.1 ml) was added. After stirring in the dark for 3 days, the mixture was separated by HPLC on a column (1 cm. diam.) of C₁₈ reversed-phase silica with a gradient of 25 to 100% MeCN in 0.01M formate buffer (pH 4.0). The principal red fraction was concentrated and rechromatographed with H₂ O in place of the formate buffer. Evaporation in vacuo afforded the product (2.1 mg) as an amorphous red powder. Electrospray MS: ion at 723 (M+H) indicates M=722. C₃₅ H₃₄ N₂ O₁₅ requires M=722.

EXAMPLE 12 N-4-Nitrobenzyloxycarbonyl-mitomycin C

A solution of mitomycin C (36 mg) in dimethyl formamide (DMF) (2 ml) containing NEt₃ 14 μl) was added to 4-nitrobenzyl chloroformate (30 mg) and the mixture was stirred at 210 for 4 hr. After evaporation in vacuo, the residue was chromatographed on a column of silica gel with EtOAc to give the title compound as a dark red solid (49 mg). Electrospray MS: ion at 514 (M+H) indicates M=513. C₂₃ H₂₃ N₅ O₉ requires M=513.

EXAMPLE 13

Formation of Actinomycin D by the Action of the Nitroreductase upon Prodrug V

V (100 μM) and cofactor (500 μM NADH) were incubated with enzyme (2 μg/ml E. coli B nitroreductase of Example 1 in 10 mM sodium phosphate buffer (pH7) in air at 37° C. At various times, aliquots (20 μl) were injected onto a Microsorb C18 reverse-phase (240×4.7 mm) HPLC column and eluted isocratically (1 ml/min) with 80% acetonitrile in water. The eluate was continuously monitored for absorption at 280 nm and the concentration of drug calculated by integration of the peak corresponding to this compound on the HPLC trace. Only when the enzyme was present was actinomycin D released from the prodrug.

Disappearance of the prodrug N-4-nitro-benzyloxycarbonyl-actinomycin D (V) and formation of actinomycin D during incubation with the E. coli B nitroreductase of Example 1 is shown in FIG. 2.

EXAMPLE 14 The Formation of Mitomycin C by the Action of the Nitroreductase Enzyme upon Prodrug VII

Prodrug VII (100 μM) and cofactor (500 μM NADH) were incubated with enzyme (5 μg/ml E. coli B nitroreductase of Example 1) in 10 mM sodium phosphate buffer (pH7) in air at 37° C. At various times aliquots (10 μl) were injected onto a Partisphere C18 reverse-phase (150×4.7mm) HPLC column and eluted isocratically (2.0 ml/min) with 50% methanol in water. The eluate was continuously monitored for absorption at 260 and 340 nm and the concentration of the drugs calculated by integration of the peak corresponding to the compound on the HPLC trace.

Disappearance of prodrug VII and the formation of mitomycin C during this incubation is shown in FIG. 3. Only in the presence of the enzyme is mitomycin C released from the prodrug.

EXAMPLE 15 Activation of Prodrug I where R¹ NH₂ =III and Prodrug V by the Nitroreductase

Generation of cytotoxicity by the action of the E. coli nitroreductase of Example 1 upon the prodrugs 4-[bis(2-chloroethyl)amino]phenylcarbamic acid 4'-nitrobenzyl ester (I where R¹ NH₂ =III) and N-4-nitrobenzyloxy-carbonyl-actinomycin D (V) are shown in Table 8.

                  TABLE 8                                                          ______________________________________                                         The effect of enzyme-activated prodrugs on the survival of V79                   cells. 1 ml volumes of V79 cells (2 × 10.sup.5 /ml) were incubated      with prodrug, 500 μM NADH, and 10 μg/ml enzyme. After a 2 hour           incubation at 37° C., the cells were harvested and assayed for          their colony forming ability, and the supernatant assayed for                  the concentration of remaining prodrug by HPLC.                                                                 % DRUG                                                                          TREATMENT % SURVIVAL REDUCTION              ______________________________________                                         CONTROL            100        --                                                 + 500 μM NADH 100 --                                                      + 50 μM (I where R.sup.1 NH.sub.2 = III)                                                       27.1       <1.0                                             + NR + 50 μM (I where R.sup.1 NH.sub.2 = III) +                                                .0.001     30                                                 500 μM NADH                                                               + 1 μM V        98.3       <1.0                                               + 10 μM V  39.3 <1.0                                                        + NR + 1 μM V +  27.4 90.5                                                  500 μM NADH                                                               + NR + 10 μM V +                                                                               .0.0001    93                                                 500 μM NADH                                                               + NR + CB 10-107 + NADH                                                                           5.0        45                                               ______________________________________                                    

EXAMPLE 16 Preparation and Binding of Antibody: Enzyme Conjugate

Antibody:enzyme conjugate of A5B7 F(ab)₂ : nitroreductase was prepared using the heterobifuncitional agents succinimidyl 4-(p-maleimidophenyl)butyrate (SMPB) to insert active maleimide groups into the immunoglobulin, and 2-mercapto-[S-acetyl]acetic acid N-hydroxysuccinimide (SATA) to thiolate the E. coli nitroreductase. On mixing the proteins the maleimide groups react with thiols to form a thioether bond. The molar ratio of SMPB:immunoglobulin used was 10:1 and the molar ratio of SATA:nitroreductase was 4:1. Antibody:enzyme conjugate thus prepared was isolated from high molecular weight aggregates and uncoupled components by gel filtration chromatography using a calibrated column (16×700mm) of Superdex G200. The column was equilibrated in phosphate buffered saline (PBS) and eluted with the same buffer at a flow rate of 1.0 ml/min. Fractions containing material corresponding in molecular weight to 2:1 and 1:1 conjugate (316 KDa and 233 KDa respectively) were pooled and rechromatographed on the same column and samples from pooled fractions were run on 4-15% SDS-PAGE gels (Pharmacia Phastgels, run for 60 vh) under non-reducing conditions together with calibration proteins. The conjugate was present as material with Mr 125 KDa (corresponding to 1:1 F(ab')₂ :nitroreductase and higher molecular weight conjugates together with less amounts of free F(ab')₂ and nitroreductase.

The enzymic activity of the conjugate was established by the routine assay using CB1954 as substrate. The conjugate was shown to bind to plates coated with 1 μg/ml CEA antigen and to contain binding sites for both rabbit anti-mouse immunoglobulin and rabbit anti-nitroreductase secondary antibodies (FIG. 4). Samples of uncoupled F(ab')₂ and nitroreductase were used to confirm the specificity of the secondary antibody binding.

The antibody binding was determined using a standard horse radish peroxidase (HRP) calorimetric ELISA assay, with the results being read at 405 nm. The inverted open triangles on FIG. 4 show that bound A5B7 F(ab')₂ -nitroreductase conjugate can be detected with a goat anti-mouse immunoglobulin antibody, and the closed inverted triangles show that the conjugate is also detected by a rabbit anti-NR antibody (the anti-NR antibody being detected via the use of a goat anti-rabbit immunoglobulin antibody. The controls shown in FIG. 4 are: closed circles=binding of unconjugated A5B7 F(ab')₂ ; open squares=A5B7 F(ab')₂ -NR conjugate detected with goat anti-rabbit immunoglobulin; closed squares=NR only detected with rabbit anti-NR; open triangles=NR only detected with goat anti-mouse immunoglobulin.

EXAMPLE 17 In Vivo Toxicity of Prodrug

The actinomycin D prodrug (AMDPD) of formula V was tested for toxicity in mice. Groups of 3 mice were given 1, 10 or 100 mg/kg body weight i.p. of AMPD, and two further groups of 3 mice were given 1 and 10 mg/kg body weight i.p. of actinomycin D (AMD) dissolved in arachis oil. A further group of 3 mice were untreated, and a final group of 3 mice were given arachis oil i.p. only.

The body weight of the mice were monitored over 9 days. The results are shown on Table 9. All mice treated with 10 mg/kg of AMD were dead by day 1. A similar result was obtained with a dose of 5 mg/kg. These data indicate that AMDPD is at least 20 to 100 fold less toxic than AMD. In practice, the toxicity of AMDPD is likely to be even lower, since the preparation of AMDPD used contains about 1% unconverted AMD.

                  TABLE 9                                                          ______________________________________                                         Toxicity of AMD and AMDPD                                                                   Weight as % of Day 0                                              Compound Dose    Day 2    Day 3  Day 7  Day 9                                  ______________________________________                                         Prodrug  100     96.4     95.6   95     96                                                      (1 dead)                                                               10      109      105    108    110                                       1.0 102 101 101 103                                                         Active Drug                                                                             10      All dead on day 1.                                                     1.0     98.6     95     101.6  103                                    Control  --      104      103    106    106                                      Oil  99 99 101 102                                                           ______________________________________                                    

    __________________________________________________________________________     #             SEQUENCE LISTING                                                    - -  - - (1) GENERAL INFORMATION:                                              - -    (iii) NUMBER OF SEQUENCES: 5                                            - -  - - (2) INFORMATION FOR SEQ ID NO:1:                                      - -      (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 1167 base - #pairs                                                 (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: double                                                       (D) TOPOLOGY: linear                                                  - -     (ii) MOLECULE TYPE: DNA (genomic)                                      - -    (iii) HYPOTHETICAL: NO                                                  - -     (iv) ANTI-SENSE: NO                                                    - -     (vi) ORIGINAL SOURCE:                                                           (A) ORGANISM: Escherichia - #coli                                     - -     (ix) FEATURE:                                                                   (A) NAME/KEY: CDS                                                              (B) LOCATION: 176..826                                                - -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:1:                                - - TCGCGATCTG ATCAACGATT CGTGGAATCT GGTGGTTGAT GGTCTGGCTA AA -             #CGCGATCA     60                                                                  - - AAAAAGAGTG CGTCCAGGCT AAAGCGGAAA TCTATAGCGC ATTTTTCTCG CT -             #TACCATTT    120                                                                  - - CTCGTTGAAC CTTGTAATCT GCTGGCACGC AAAATTACTT TCACATGGAG TC - #TTT        ATG     178                                                                                        - #                  - #                  - #              Met                                                                                                - #                  - #                  - #                - - GAT ATC ATT TCT GTC GCC TTA AAG CGT CAT TC - #C ACT AAG GCA TTT GAT           226                                                                        Asp Ile Ile Ser Val Ala Leu Lys Arg His Se - #r Thr Lys Ala Phe Asp                          5    - #              10    - #              15                   - - GCC AGC AAA AAA CTT ACC CCG GAA CAG GCC GA - #G CAG ATC AAA ACG CTA           274                                                                        Ala Ser Lys Lys Leu Thr Pro Glu Gln Ala Gl - #u Gln Ile Lys Thr Leu                     20         - #         25         - #         30                       - - CTG CAA TAC AGC CCA TCC AGC ACC AAC TCC CA - #G CCG TGG CAT TTT ATT           322                                                                        Leu Gln Tyr Ser Pro Ser Ser Thr Asn Ser Gl - #n Pro Trp His Phe Ile                 35             - #     40             - #     45                           - - GTT GCC AGC ACG GAA GAA GGT AAA GCG CGT GT - #T GCC AAA TCC GCT GCC           370                                                                        Val Ala Ser Thr Glu Glu Gly Lys Ala Arg Va - #l Ala Lys Ser Ala Ala             50                 - # 55                 - # 60                 - # 65        - - GGT AAT TAC GTG TTC AAC GAG CGT AAA ATG CT - #T GAT GCC TCG CAC GTC           418                                                                        Gly Asn Tyr Val Phe Asn Glu Arg Lys Met Le - #u Asp Ala Ser His Val                             70 - #                 75 - #                 80               - - GTG GTG TTC TGT GCA AAA ACC GCG ATG GAC GA - #T GTC TGG CTG AAG CTG           466                                                                        Val Val Phe Cys Ala Lys Thr Ala Met Asp As - #p Val Trp Leu Lys Leu                         85     - #             90     - #             95                   - - GTT GTT GAC CAG GAA GAT GCC GAT GGC CGC TT - #T GCC ACG CCG GAA GCG           514                                                                        Val Val Asp Gln Glu Asp Ala Asp Gly Arg Ph - #e Ala Thr Pro Glu Ala                    100          - #       105          - #       110                       - - AAA GCC GCG AAC GAT AAA GGT CGC AAG TTC TT - #C GCT GAT ATG CAC CGT           562                                                                        Lys Ala Ala Asn Asp Lys Gly Arg Lys Phe Ph - #e Ala Asp Met His Arg                115              - #   120              - #   125                           - - AAA GAT CTG CAT GAT GAT GCA GAG TGG ATG GC - #A AAA CAG GTT TAT CTC           610                                                                        Lys Asp Leu His Asp Asp Ala Glu Trp Met Al - #a Lys Gln Val Tyr Leu            130                 1 - #35                 1 - #40                 1 -       #45                                                                               - - AAC GTC GGT AAC TTC CTG CTC GGC GTG GCG GC - #T CTG GGT CTG GAC         GCG      658                                                                     Asn Val Gly Asn Phe Leu Leu Gly Val Ala Al - #a Leu Gly Leu Asp Ala                           150  - #               155  - #               160               - - GTA CCC ATC GAA GGT TTT GAC GCC GCC ATC CT - #C GAT GCA GAA TTT GGT           706                                                                        Val Pro Ile Glu Gly Phe Asp Ala Ala Ile Le - #u Asp Ala Glu Phe Gly                        165      - #           170      - #           175                   - - CTG AAA GAG AAA GGC TAC ACC AGT CTG GTG GT - #T GTT CCG GTA GGT CAT           754                                                                        Leu Lys Glu Lys Gly Tyr Thr Ser Leu Val Va - #l Val Pro Val Gly His                    180          - #       185          - #       190                       - - CAC AGC GTT GAA GAT TTT AAC GCT ACG CTG CC - #G AAA TCT CGT CTG CCG           802                                                                        His Ser Val Glu Asp Phe Asn Ala Thr Leu Pr - #o Lys Ser Arg Leu Pro                195              - #   200              - #   205                           - - CAA AAC ATC ACC TTA ACC GAA GTG TAATTCTCTC TT - #GCCGGGCA TCTGCCCGGC          856                                                                        Gln Asn Ile Thr Leu Thr Glu Val                                                210                 2 - #15                                                     - - TATTTCCTCT CAGATTCTCC TGATTTGCAT AACCCTGTTT CAGCCGTCAT CA -              #TAGGCTGC    916                                                                  - - TGTTGTATAA AGGAGACGTT ATGCAGGATT TAATATCCCA GGTTGAAGAT TT -             #AGCGGGTA    976                                                                  - - TTGAGATCGA TCACACCACC TCGATGGTGA TGATTTTCGG TATTATTTTT CT -             #GACCGCCG   1036                                                                  - - TCGTGGTGCA TATTATTTTG CATTGGGTGG TACTGCGGAC CTTCGAAAAA CG -             #TGCCATCG   1096                                                                  - - CCAGTTCACG GCTTTGGTTG CAAATCATTA CCCAGAATAA ACTCTTCCAC CG -             #TTTAGCTT   1156                                                                  - - TTACCCTGCA G               - #                  - #                       - #     1167                                                                   - -  - - (2) INFORMATION FOR SEQ ID NO:2:                                      - -      (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 217 amino - #acids                                                 (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                  - -     (ii) MOLECULE TYPE: protein                                            - -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:2:                                - - Met Asp Ile Ile Ser Val Ala Leu Lys Arg Hi - #s Ser Thr Lys Ala Phe         1               5 - #                 10 - #                 15               - - Asp Ala Ser Lys Lys Leu Thr Pro Glu Gln Al - #a Glu Gln Ile Lys Thr                    20     - #             25     - #             30                   - - Leu Leu Gln Tyr Ser Pro Ser Ser Thr Asn Se - #r Gln Pro Trp His Phe                35         - #         40         - #         45                       - - Ile Val Ala Ser Thr Glu Glu Gly Lys Ala Ar - #g Val Ala Lys Ser Ala            50             - #     55             - #     60                           - - Ala Gly Asn Tyr Val Phe Asn Glu Arg Lys Me - #t Leu Asp Ala Ser His        65                 - # 70                 - # 75                 - # 80        - - Val Val Val Phe Cys Ala Lys Thr Ala Met As - #p Asp Val Trp Leu Lys                        85 - #                 90 - #                 95               - - Leu Val Val Asp Gln Glu Asp Ala Asp Gly Ar - #g Phe Ala Thr Pro Glu                   100      - #           105      - #           110                   - - Ala Lys Ala Ala Asn Asp Lys Gly Arg Lys Ph - #e Phe Ala Asp Met His               115          - #       120          - #       125                       - - Arg Lys Asp Leu His Asp Asp Ala Glu Trp Me - #t Ala Lys Gln Val Tyr           130              - #   135              - #   140                           - - Leu Asn Val Gly Asn Phe Leu Leu Gly Val Al - #a Ala Leu Gly Leu Asp       145                 1 - #50                 1 - #55                 1 -       #60                                                                               - - Ala Val Pro Ile Glu Gly Phe Asp Ala Ala Il - #e Leu Asp Ala Glu         Phe                                                                                              165  - #               170  - #               175              - - Gly Leu Lys Glu Lys Gly Tyr Thr Ser Leu Va - #l Val Val Pro Val Gly                   180      - #           185      - #           190                   - - His His Ser Val Glu Asp Phe Asn Ala Thr Le - #u Pro Lys Ser Arg Leu               195          - #       200          - #       205                       - - Pro Gln Asn Ile Thr Leu Thr Glu Val                                           210              - #   215                                                  - -  - - (2) INFORMATION FOR SEQ ID NO:3:                                      - -      (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 217 amino - #acids                                                 (B) TYPE: amino acid                                                           (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                  - -     (ii) MOLECULE TYPE: protein                                            - -    (iii) HYPOTHETICAL: NO                                                  - -     (iv) ANTI-SENSE: NO                                                    - -     (vi) ORIGINAL SOURCE:                                                           (A) ORGANISM: Salmonella - #typhimurium                               - -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:3:                                - - Met Asp Ile Val Ser Val Ala Leu Gln Arg Ty - #r Ser Thr Lys Ala Phe       1               5   - #                10  - #                15                - - Asp Pro Ser Lys Lys Leu Thr Ala Glu Glu Al - #a Asp Lys Ile Lys Thr                   20      - #            25      - #            30                    - - Leu Leu Gln Tyr Ser Pro Ser Ser Thr Asn Se - #r Gln Pro Trp His Phe               35          - #        40          - #        45                        - - Ile Val Ala Ser Thr Glu Glu Gly Lys Ala Ar - #g Val Ala Lys Ser Ala           50              - #    55              - #    60                            - - Ala Gly Asn Tyr Thr Phe Asn Glu Arg Lys Me - #t Leu Asp Ala Ser His       65                  - #70                  - #75                  - #80         - - Val Val Val Phe Cys Ala Lys Thr Ala Asn As - #p Asp Ala Trp Leu Glu                       85  - #                90  - #                95                - - Arg Val Val Asp Gln Glu Asp Ala Asp Gly Ar - #g Phe Ala Thr Pro Glu                   100      - #           105      - #           110                   - - Ala Lys Ala Ala Asn Asp Lys Gly Arg Arg Ph - #e Phe Ala Asp Met His               115          - #       120          - #       125                       - - Arg Val Ser Leu Lys Asp Asp His Gln Trp Me - #t Ala Lys Val Val Tyr           130              - #   135              - #   140                           - - Leu Asn Val Gly Asn Phe Leu Leu Gly Val Al - #a Ala Met Gly Leu Asp       145                 1 - #50                 1 - #55                 1 -       #60                                                                               - - Ala Val Pro Ile Glu Gly Phe Asp Ala Glu Va - #l Leu Asp Ala Glu         Phe                                                                                              165  - #               170  - #               175              - - Gly Leu Lys Glu Lys Gly Tyr Thr Ser Leu Va - #l Val Val Pro Val Gly                   180      - #           185      - #           190                   - - His His Ser Val Glu Asp Phe Asn Ala Gly Le - #u Pro Lys Ser Arg Leu               195          - #       200          - #       205                       - - Pro Leu Glu Thr Thr Leu Thr Glu Val                                           210              - #   215                                                  - -  - - (2) INFORMATION FOR SEQ ID NO:4:                                      - -      (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 31 amino - #acids                                                  (B) TYPE: amino acid                                                           (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                  - -     (ii) MOLECULE TYPE: protein                                            - -    (iii) HYPOTHETICAL: NO                                                  - -     (iv) ANTI-SENSE: NO                                                    - -      (v) FRAGMENT TYPE: N-terminal                                         - -     (vi) ORIGINAL SOURCE:                                                           (A) ORGANISM: Escherichia - #coli                                     - -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:4:                                - - Met Asp Ile Ile Ser Val Ala Leu Lys Arg Hi - #s Ser Thr Lys Ala Phe       1               5   - #                10  - #                15                - - Asp Ala Ser Lys Xaa Leu Thr Pro Glu Gln Al - #a Asp Gln Ile Lys                       20      - #            25      - #            30                    - -  - - (2) INFORMATION FOR SEQ ID NO:5:                                      - -      (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 41 amino - #acids                                                  (B) TYPE: amino acid                                                           (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                  - -     (ii) MOLECULE TYPE: peptide                                            - -    (iii) HYPOTHETICAL: NO                                                  - -     (iv) ANTI-SENSE: NO                                                    - -      (v) FRAGMENT TYPE: N-terminal                                         - -     (vi) ORIGINAL SOURCE:                                                           (A) ORGANISM: Escherichia - #coli                                     - -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:5:                                - - Ala Lys Gln Val Tyr Leu Asn Val Gly Asn Ph - #e Leu Leu Gly Val Ala       1               5   - #                10  - #                15                - - Ala Leu Gly Leu Asp Ala Val Pro Ile Glu Gl - #y Phe Asp Ala Ala Ile                   20      - #            25      - #            30                    - - Leu Asp Ala Glu Phe Gly Leu Lys Ile                                               35          - #        40                                             __________________________________________________________________________ 

What is claimed is:
 1. A method of reducing the nitro group of a p-nitrophenyl-benzyloxycarbonyl residue which method comprises:bringing a compound comprising a p-nitrophenyl-benzyloxycarbonyl group into contact with a bacterial nitroreductase enzyme which has the following characteristics:(i) it is a flavoprotein having a molecular weight in the range 20-60 Kilodaltons; (ii) it requires either NADH or NAD(P)H or analogues thereof as a cofactor; (iii) it has a Km for NADH or NAD(P)H in the range of 1-100 μM; and it is capable of reducing either or both nitro groups of CB1954 and analogues thereof to a cytotoxic form;under conditions in which the nitro group of said p-nitrophenyl-benzyloxycarbonyl residue is reduced by the action of said enzyme.
 2. A method according to claim 1 wherein said nitroreductase comprises the amino acid sequence of SEQ ID NO:2.
 3. A method according to claim 1 wherein said nitroreductase is an Escherichia coli nitroreductase having a molecular weight of about 24 Kilodaltons and requiring a cofactor selected from the group consisting of NADH, NAD(P)H, and analogues thereof.
 4. A method according to claim 2 wherein said Escherichia coli is Escherichia coli B.
 5. A method according to claim 1 wherein said p-nitrophenyl-benzyloxycarbonyl residue is a moiety of a compound of formula (I), (II), (V), (VI) or (VII): ##STR12## wherein R¹ and R² are groups such that the compounds R¹ NH₂ and R² OH are cytotoxic compounds.
 6. A method according to claim 5, wherein said compound R¹ NH₂ is a nitrogen mustard of formula (IV) ##STR13## wherein R' and R" are H, F, or Cl.
 7. A method according to claim 5 wherein R' and R" are both H.
 8. A method according to claim 5 wherein R² OH is a phenolic nitrogen mustard compound.
 9. A method according to claim 5 wherein said compound is of the formula (VIII): ##STR14##10.
 10. A method according to claim 38 which is performed in the presence of cells such that reduction of said p-nitrophenyl-benzyloxycarbonyl residue converts a compound comprising said residue into a compound cytotoxic to said cells.
 11. A method according to claim 10 wherein said cells, following exposure to said cytotoxic compound, are harvested and assayed for their colony forming ability. 